Mnemosyne is memory personified, a Titaness who by lying with Zeus for nine nights becomes the mother of the nine Muses, so that in Greek thought all art, poetry and knowledge are the children of remembrance, a genealogy that says something about a largely oral culture in which the poet's craft was the preservation and re-performance of the past. Her role reaches beyond poetry into eschatology.
The Orphic gold leaves, thin inscribed tablets buried with initiates in southern Italy and Crete from the fourth century BCE, instruct the dead soul to avoid the spring of Lethe, forgetfulness, and to drink instead from the pool of Mnemosyne, Memory, so as to keep its wits and its identity among the dead and escape the cycle of rebirth. Memory, in that scheme, is what saves.
Beyond the Orphic gold leaves, Mnemosyne's name survives directly in English and other modern languages through scientific and technical vocabulary derived from Greek mneme, memory, including "mnemonic" and "amnesia," a linguistic afterlife considerably more visible today than any trace of ancient cult, since no temple or festival dedicated to her as an independent goddess is securely attested, her worship, where it existed at all, apparently folded into that of her daughters the Muses.